1. Field of the Invention
The present disclosure is directed at a method for improving charge uniformity of a developer mix across various temperature and humidity conditions by performing the step of modifying the surface of the magnetic carrier particle by treating the surface of the magnetic carrier particle with surface additive(s) before the magnetic carrier particle is mixed with the toner resin particle to form the developer mix.
2. Description of the Related Art
Toners for use in electrophotographic printers include two primary types, mechanically milled toners and chemically prepared toners (CPT). Chemically prepared toners have significant advantages over mechanically milled toners including better print quality, higher toner transfer efficiency and lower torque properties for various components of the electrophotographic printer such as a developer roller, a fuser belt and a charge roller. The particle size distribution of CPTs is typically narrower than the particle size distribution of mechanically milled toners. The size and shape of CPTs are also easier to control than mechanically milled toners.
There are several known types of CPT including suspension polymerization toner, emulsion aggregation toner, latex aggregation toner, toner made from a dispersion of pre-formed polymer in solvent and chemically milled toner. While emulsion aggregation toner requires a more complex process than other CPTs, the resulting toner has a relatively narrower size distribution. Emulsion aggregation toners can also be manufactured with a smaller particle size allowing improved print resolution. The emulsion aggregation process also permits better control of the shape and structure of the toner particles which then allows the toner particles to be tailored to fit the desired cleaning, doctoring and transfer properties. The shape of the toner particles produced from an emulsion aggregation process may be optimized to ensure proper and efficient cleaning of the toner from various electrophotographic printer components, such as the developer roller, charge roller and doctoring blades, in order to prevent filming or unwanted deposition of toner on these components.
Toner may be utilized in image forming devices, such as printers, copiers and fax machines, to form images on a sheet of media. The image forming apparatus transfers the toner from a reservoir to the media via a developer system utilizing differential charges generated between the toner particles and the various components in the developer system. Electrophotographic printing can be carried out using a monocomponent development (MCD) system that requires the use of a toner adder roll, developer roll, and doctor blade for charging and doctoring the toner. Alternatively, the electrophotographic printing can be carried out using a dual component development (DCD) system which requires the use of a magnetic carrier particle and a magnetic roll to help charge the toner. Using a DCD system has the advantage of using fewer components and allowing for longer life cartridges and hence, a lower cost per page. Regardless of whether the toner is charged using a MCD or a DCD process, the printing of toner uses the same process of toner transfer to an imaging substrate that has been discharged via light, such as a photoconductor or photoreceptor drum or belt. Toner is then directly transferred to a media sheet or to an intermediate image transfer member before being transferred onto a media sheet.
In DCD printing, a mixture of toner particles and magnetic carrier particles is referred to as a developer mix. Mixing magnetic carrier particles with the surface-treated toner particles in the presence of some electrical voltage generates a triboelectric charge. It is desirable to have the developer mix maintain a uniform triboelectric charge across varying temperature and humidity conditions, including hot/wet (78° F./80% relative humidity), cold/dry (60° F./8% relative humidity) and ambient (72° F./40% relative humidity). The uniform tribolelectric charging behavior of the developer mix supplies a uniform amount of toner to magnetic roller and subsequently to the photoconductor drum. Therefore, the print quality thus obtained is similar across various environments and does not change as a function of temperature and/or humidity.
However using toner particles in a developer mix manufactured via an emulsion aggregation usually results in the developer mix having an undesirable variable charge across different temperature and humidity conditions. This is due to the fact that the emulsion aggregation process of making toner is a wet process that involves the use of flocculants such as metal salts, acids, and bases. The emulsion aggregation process also uses surfactants and/or dispersants in resin, pigment and wax emulsions. Insufficient removal of these surfactants or dispersants can have a significant negative impact on the tribocharge of the developer mix because the presence of acid, base or salts can negatively influence the tribocharging nature. For example, the presence of a trivalent salt such as aluminum chloride or aluminum sulfate can significantly lower the triboelectric charging behavior, in particular if it is a negative charging system. Additionally, the presence of salts on toner surface can negatively influence the interaction of the toner with moisture, thereby rendering the system humidity-sensitive.
Accordingly, it is desirous to achieve uniform tribocharge behavior for a developer mix from various toner batches and reproducibility across various manufacturing toner lots as well as across different temperature and humidity conditions. This tribocharge uniformity ultimately leads to uniform print quality throughout the life of the cartridge and not dependent on the varying environmental temperature and conditions.